implicit_plot leaves out points

edit

I'm trying to use implicit_plot to graph nullclines for a competition model, but only some parts of the nullclines are plotted. I have:

var("N,P")
Nprime(N,P) = N*(3-N-2*P)
implicit_plot(Nprime, (N,0,3), (P,0,3))

Clearly, this function is 0 at all points where N=0, but the graph only shows them up to P=3/2. Is there anything I can do to avoid this or is it a bug in implicit_plot?